Land vehicles, including automotive vehicles, and in particular heavy-duty vehicles such as logging trucks and trucks used on construction sites, when launched or started off under off-road and similar conditions, can be stuck because the drive wheels experience reduced traction on the ground. Experienced drivers have developed strategies for solving this problem, such as engaging a lateral differential lock, or engaging a longitudinal differential lock, or lifting a set of wheels so that the weight on each remaining wheel is increased. A more elaborate method, known as rocking, involves intermittently transmitting power to the drive wheels. More specifically, the driver repeats a sequence in which the power transmitted to the drive wheels is first increased until the wheels spin, and subsequently releases the accelerator pedal and disengages the main clutch. An even more elaborate version of the method involves alternately changing from a forward gear ratio to a reverse gear ratio.
Several other ground conditions can present similar challenges. For example, if the vehicle comes to rest with one or more wheels in a trough, ditch or similar depression, engine power can be insufficient to launch the vehicle out of the depression and into productive travel. Similarly, if a log, ridge, mound or other impediment is located ahead of one or more of the vehicle's wheel (s) prior to launch, similar difficulties can be experienced getting the vehicle to launch into travel. In a more generic sense, these several start-up challenging conditions can be collectively referred to as impedances or impediments to the initiation or continuation of travel of the land vehicle.
With regard to the above-described rocking methods for initiating vehicle launch, great skill and experience is required since the driver, seated in the cab of the vehicle has little information about the real state of wheel spin and has to react quickly to achieve the desired effects. For this reason, novice drivers, at least with respect to these more difficult driving conditions, are often left at a disadvantage when compared to their more experienced counterparts. Moreover, these methods are often found to be incompatible with transmission and traction control systems utilized in modern heavy-duty vehicles.
Many semi-automatic transmission systems include means for automatically selecting a predetermined start-up gear when the vehicle is started up from a standstill condition. However, it has been suggested in U.S. Pat. No. 6,383,116 to allow the driver to operate the gear-selector lever for selecting a higher start-up gear than the predetermined start-up gear that would otherwise be automatically selected by the electronic controller. This method can be appropriate when traction is reduced on a non-deformable road surface, for instance, due to ice or a thin layer of snow. Such a strategy, however, is of little or no use when the ground surface is deformable, such as in the woods or on a construction site because in those cases the drive wheels must be allowed to churn through the deformable surface for finding traction on more solid, often underlying, ground.
It has further been suggested in WO 02/04242 to adapt a vehicle traction control system for operation upon a deformable ground surface by increasing the amount of slip permitted by the traction control system for the drive wheels and thereby allow the wheels to churn through the deformable surface. The vehicle traction control system is actuated when drive wheel slip exceeds a predetermined slip threshold in order to reduce the amount of drive wheel slip by reducing drive wheel speed. The traction control system also monitors vehicle deceleration. Upon detecting that the vehicle is decelerating and that the vehicle deceleration has exceeded a deceleration threshold, the traction control determines that the vehicle has encountered a deformable surface such as mud or deep snow. Upon detecting entry onto a deformable surface, the slip threshold is linearly increased over period of time to increase the drive wheel speed. This control routine does not, however, provide a method for starting-up from standstill.